The invention relates to firing pulse generators for thyristors, and similar types of static electric valves such as used for power conversion in polyphase electrical supply. Digital firing pulse generators are known from "Thyristor Phase-Controlled Converters and Cycloconverters" by B. R. Pelly, chapter ten, pages 248-277 - John Wiley and Sons 1971. Of importance with such systems is the reliability and accurcy of the control pulses. Phase-locked oscillator techniques have been used in order to obtain pulse timing which is independent from actual fluctuations in the power lines, so that, for the purpose of establishing an accurate firing angle, the firing instant is fixed in electrical degrees. Thus, U.S. Pat. No. 3,534,285 entitled "Digital Phase Control Circuit for Synchronizing an Oscillator to a Harmonic Or a Reference Frequency" by P. J. Kobold et al issued Oct. 13, 1970 teaches a system where a signal is applied to a phase detector which in turn controls a voltage controlled oscillator that generates a signal applied to a counter for providing a binary output.
Such a voltage controlled oscillator when synchronized with the phase line of a polyphase AC power system, as described in U.S. Pat. No. 3,891,912 of Watanabe, may provide a binary output to be compared with a voltage reference signal for generating firing pulses, at selected firing angles, to thyristors connected in a polyphase arrangement on the power lines. The thyristors in the Watanabe Patent are fired in succession by a ring counter triggered on each comparison. At the same time, the binary count in the counter is modified in its most significant digits so as to generate in a ring fashion, upon each triggering by the comparator, a binary representation of the subsequent phase of the power lines in which the next controlled rectifier to be fired is connected. However, in the Watanabe system, should a lack of concordance occur during transition of the ring counter, the binary counter, or both, a proper correspondency between the phase represented by the digital count and the selected rectifier to be fired no longer exists. Improper firings will result.
In order to overcome this drawback, the present invention proposes that the comparator trigger only the ring counter and that the ring counter be locked with the digital counter used for the determination of the firing angle and the generation of a triggering pulse.